The use of electronic trip units within low-ampere industrial rated circuit breakers has heretofore been forestalled because of size constraints on the discreet electrical and electromagnetic components within the circuit breaker enclosure. The current transformer used in conjunction with the electronic trip unit, has a two-fold requirement namely, to provide an input signal to the trip unit representative of the current flow within the associated protected power circuit while providing the necessary input power to the trip unit power supply. A predetermined maximum core volume is required within the current transformer to ensure that the current transformer does not become magnetically saturated upon the occurrence of overcurrent conditions when used within compact circuit breakers having variable ampere ratings while a predetermined minimum core volume insures that the core will become sufficiently magnetized at the lower steady-state operating current levels.
With earlier-designed electronic trip circuit breakers, such as described within U.S. Pat. No. 4,281,359, for example, a standard trip unit circuit is employed over a wide range of ampere ratings while the size of the current transformer used to sense the input current to the trip unit circuit is correspondingly increased in proportion to the increased ampere rating.
When compact electronic trip unit circuit breakers employing various accessory devices, such as described in U.S. Pat. No. 4,754,247, are used within industrial-rated power distribution circuits, the size constraints of the circuit breaker enclosure limit the geometry of the current transformer core to a size just sufficient to provide operating power to the electronic trip unit circuit without becoming saturated at the higher ampere ratings due to the low inductance of the smaller core. Another problem involved with the use of small-sized current transformer cores is the lack of sufficient core inductance to provide the requisite core magnetization for transformer operation at the lower ampere ratings.
One purpose of the instant invention accordingly, is to provide an electronic trip circuit breaker for industrial power distribution circuits whereby a single current transformer core size fits within the circuit breaker enclosure and supplies sufficient sensing current and operating power to the trip unit circuit over a wide range of circuit breaker ampere ratings.